1. Field of the Invention
The present invention relates to a high-voltage power supply circuit for effecting rectification and successive multiplication of an ac input and outputting a high-voltage direct current, and more particularly to a high-voltage power supply circuit suitable for supplying power to a traveling-wave tube that requires high voltage at a relatively low load current.
2. Description of the Related Art
Referring now to FIG. 1, an example of the prior art of this type of high-voltage power supply circuit comprises: transformer 100; diode bridges 111, 112, and 113; and capacitors C101, C102, and C103. Transformer 100 includes: primary winding 101 to which an ac voltage is applied; and first, second and third secondary windings 102, 103, and 104 formed from a number of turns according to the desired step-up voltage. Diode bridges 111, 112, and 113 effect full-wave rectification of the ac voltage outputted from first, second, and third secondary windings 102, 103, and 104. Capacitors C101-C103 reduce ripple in the dc voltage outputted from diode bridges 111-113, respectively. The output terminals of diode bridges 111-113 are connected to each other in series.
As an example, traveling-wave tube 120, which requires high voltage at a relatively low load current, is connected as the load to the high-voltage power supply circuit with the aforementioned type of construction shown in FIG. 1.
In other words, cathode terminal 121 of traveling-wave tube 120 is connected to one output terminal of diode bridge 111, and collector terminal 122 of traveling-wave tube 120 is connected both to the other output terminal of diode bridge 111 and to one output terminal of diode bridge 112. Collector terminal 123 of traveling-wave tube 120 is connected both to the other output terminal of diode bridge 112 and to one output terminal of diode bridge 113. Finally, the other output terminal of diode bridge 113 is connected to helix terminal 124 of traveling-wave tube 120.
The load current hereupon passes through collector terminal 122, collector terminal 123, and helix terminal 124 of traveling-wave tube 120 and flows to cathode terminal 121.
To obtain the high voltages that are applied to each of cathode terminal 121, collector terminal 122, and collector terminal 123 of traveling-wave tube 120 in the high-voltage power supply circuit shown in FIG. 1, transformer 100 includes three secondary windings 102, 103, and 104 corresponding to the applied voltages.
The turn ratio of the transformer is equal to the ratio of the voltage applied to the primary winding to the voltage generated at the secondary windings, and the greater the turn ratio or the greater the number of secondary windings, the larger the winding width or winding thickness required.
In addition, to allow use of the transformer at high voltages, a prescribed distance must be ensured between windings in the configuration shown in FIG. 1 to provide a high withstand voltage, and the bobbin or core of the transformer therefore must be made large. Increasing the size of the transformer core, however, both increases loss and reduces the transmission efficiency of the transformer.
Increasing the distance between windings, moreover, increases parasitic capacitance and also increases the spike voltage that occurs when diodes turn on or turn off.
Components such as the transformer, diodes, and capacitors used in the high-voltage power supply circuit of the prior art shown in FIG. 1 must have a high withstand voltage, and this results in increased circuit size and increased costs.
As one example for a solution, Japanese Patent Laid-open No. 77059/87 discloses the high-voltage power supply circuit shown in FIG. 2.
The high-voltage power supply circuit shown in FIG. 2 has one secondary winding of the transformer and a plurality (five in FIG. 2) of diode bridges at the winding ends of the secondary winding, these diode bridges being connected in series with interposed capacitors. Further, the output terminals of each diode bridge for outputting dc voltages are connected together in series as in the circuit of FIG. 1.
In addition, load R.sub.L is connected between output terminal E.sub.0 of the diode bridge that is farthest of the plurality of diode bridges connected in series from the secondary winding of the transformer and the output terminal of the diode bridge that is closest to the secondary winding of the transformer.
With this configuration, a plurality of differing high voltages can be obtained from the connection points of the diode bridges even with only one secondary winding, and the circuit therefore can be used as the power supply of a traveling-wave tube that requires simultaneous supply of power to a plurality of loads.
In the high-voltage power supply circuit shown in FIG. 2, however, the grounding of the midpoint of the secondary winding of the transformer and the output terminal of the diode bridge that is closest to the secondary winding of the transformer results in a configuration having two Cockcroft-Walton circuits (see FIG. 3).
A Cockcroft-Walton circuit is a multiple-voltage circuit that effects half-wave rectification of the ac voltage E.sub.in outputted from the secondary winding by means of capacitors C.sub.1, C.sub.2, . . . C.sub.N, and diodes D.sub.1, D.sub.2, . . . D.sub.N. The circuit therefore has the problems of lower utilization ratio of the transformer and greater output voltage ripple than the high-voltage power supply circuit shown in FIG. 1 that effects full-wave rectification.